
class Solution {
    int sum = 0;

    public TreeNode convertBST(TreeNode root) {
        if (root == null)
            return null;
            
        convertBST(root.right);
        
        sum += root.val;
        root.val = sum;
        //System.out.println(root.val + " " + sum);
        convertBST(root.left);

        return root;
    }
}